How Many Senses Do We Really Have?

“We interact with our environments so effectively and so effortlessly, it is difficult to appreciate the extensive computations that underlie even the simplest sensory experience.”​—SENSORY EXOTICA—​A WORLD BEYOND HUMAN EXPERIENCE.

PICTURE yourself cycling along a quiet country road. As you pedal, sensors in your legs enable you to apply just the right pressure to maintain your speed. Your organs of balance keep you upright; your nostrils smell the aromas; your eyes absorb the panorama; your ears are attuned to the chirping of birds. Thirsty, you grasp your drink bottle, aided by touch receptors in your fingers. Your taste buds and hot-and-cold sensors reveal the flavor of the liquid and its temperature. Sensors in your skin and those attached to your body hair tell how strong the breeze is and, in cooperation with your eyes, how fast you are going. Your skin also informs you of the ambient temperature and humidity, while your awareness of time tells you approximately how long you have been on the road. Eventually, internal senses will compel you to rest and to eat. Yes, life truly is a superb symphony of the senses!

Just Five Senses?

During such a bicycle ride, how many senses come into play​—just the traditional five: sight, hearing, smell, taste, and touch? According to the Encyclopædia Britannica, these five senses were enumerated by the ancient philosopher Aristotle, whose “influence has been so enduring that many people still speak of the five senses as if there were no others.”

However, according to Britannica, studies in skin sensitivity alone “yield evidence that the human senses number more than five.” How can that be? Certain functions once lumped together under touch are now regarded as senses in their own right. For instance, pain receptors respond to and distinguish between mechanical, thermal, and chemical forces or agents. Other sensors signal an itch. Evidence suggests that we have at least two kinds of pressure sensors​—one for light surface pressure, another for deep stimulation. Our body also has a broad range of internal senses. What is their role?

The Internal Senses

Internal senses detect changes taking place inside our body. They signal things like hunger, thirst, fatigue, internal pain, and the need to breathe or to go to the toilet. In cooperation with our biological clock, internal sensors make us feel tired at day’s end and jet-lagged if we have flown across time zones. In fact, because we can consciously “sense” the flow of time, it has been suggested that time awareness be added to the catalog of senses.

We also have a vestibular sense, or sense of balance, which is located in our inner ear. It responds to gravity, acceleration, and rotation. And finally we have a kinesthetic sense, which enables us to detect muscle tension and, even with eyes closed, the movement and position of our limbs.

Of course, sensory perception is not unique to humans. Animals also possess a broad variety of senses, including some truly astonishing ones that we do not have. In the following article, we will examine some of these. We will also take a closer look at ourselves and the unique attributes that give humans a special place among earth’s living things.

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The Wonder of Human Touch

The human hand has a particularly refined sense of touch. According to Smithsonian magazine, researchers found that our hand can detect a dot just three microns high. (A human hair has a diameter of 50 to 100 microns.) However, by “using a texture rather than a dot, the researchers found the hand can detect roughness just 75 nanometers high”​—a nanometer being one thousandth of a micron! Such remarkable sensitivity is attributed to about 2,000 touch receptors in each fingertip.

Our sense of touch also plays a key role in our health and well-being. “The caress of another person releases hormones that can ease pain and clear the mind,” says U.S.News & World Report. Some believe that when a child is deprived of the loving touch of others, its growth will be impeded.

Amazing Senses in the Animal World

SCAMPERING about looking for food, the mouse feels safe in the darkness. But it does not anticipate the pit viper’s ability to “see” the heat radiating from the mouse’s warm body​—a fatal misjudgment. A flounder lies completely concealed under a layer of sand in a shark pool, where a hungry shark is cruising in its general direction. The shark cannot see the flounder; yet, in the blink of an eye, the shark stops, plunges its nose into the sand, and devours its quarry.

Yes, the pit viper and the shark are examples of animals with specialized senses that humans do not have. On the other hand, many creatures have senses that are like our own but are more acute or able to capture a different range of perception. Eyes are a good example of this.

Eyes That See a Different World

The range of colors our eyes capture is but a minute fraction of the electromagnetic spectrum. For instance, our eyes cannot see infrared radiation, which has a longer wavelength than red light. However, pit vipers have two small organs, or pits, between their eyes and nostrils that detect infrared radiation.a Hence, even in the dark they can accurately strike at warm-blooded prey.

Beyond the violet end of the visible spectrum is ultraviolet (UV) light. Although unseen to our eyes, UV light is visible to many creatures, including birds and insects. Bees, for instance, orient themselves in relation to the sun​—even on a partly cloudy day when it is hidden—​by locating some blue sky and seeing the pattern formed by polarized UV light. Many flowering plants present patterns visible only in the UV range, and some flowers even have a “nectar marker”​—a section with a contrasting UV reflectance—​to point insects to the nectar. Certain fruits and seeds advertise themselves to birds in a similar way.

Because birds see in the UV range and because this light gives their plumage extra radiance, birds probably look more colorful to one another than to us. They have a visual “depth of richness that we can’t begin to imagine,” said one ornithologist. The ability to see UV light may even help certain hawks and kestrels to locate voles, or field mice. How so? Male voles, says the journal BioScience, “produce urine and feces containing chemicals that absorb UV, and mark their trails with urine.” Thus, birds can “identify areas of high vole density” and focus their efforts there.

Why Do Birds See So Well?

Bird vision is a marvel. “The chief reason,” says the book All the Birds of the Bible, “is that the image-forming tissue lining the eye’s interior is richer in visual cells than the eye of other creatures. The number of visual cells determines the ability of the eye to see small objects at a distance. While the retina of a man’s eye contains some 200,000 visual cells per square millimeter, most birds have three times that number, and hawks, vultures, and eagles have a million or more per square millimeter.” Additionally, some birds have the extra asset of two foveae​—areas of maximum optical resolution—​per eye, giving them a superior perception of distance and speed. Birds that catch flying insects are similarly endowed.

Birds also have an unusually soft lens that enables rapid focus. Imagine how dangerous life on the wing would be​—especially in forests and thickets—​if everything were a blur. Yes, what wisdom is manifest in the design of the avian eye!b

The Electric Sense

The scenario mentioned earlier involving the hidden flounder and the shark actually occurred during a scientific study of sharks. The researchers wanted to know if sharks and rays sensed the minute electric fields that emanate from living fish.c To find out, they hid electrodes in the sandy floor of the shark pool and applied the appropriate voltage. The result? As soon as the shark neared the electrodes, it viciously attacked them.

Sharks possess what is called passive electroreception; they sense electric fields just as the ear passively hears sound. But electric fish have active electroreception. Like a bat that emits an acoustic signal and reads the echo, these fish emit electric waves or pulses, depending on the species, and then, with special receptors, detect any disturbances made to these fields.d Thus electric fish can identify obstacles, potential prey, or even a mate.

A Built-in Compass

Think what life would be like if your body were equipped with a built-in compass. Getting lost would surely not be a problem! Within the body of a number of creatures, including honeybees and trout, scientists have found microscopic crystals of magnetite, or lodestone, a natural magnetic substance. The cells containing these crystals are connected to the nervous system. Hence, bees and trout have demonstrated the ability to detect magnetic fields. In fact, bees use the earth’s magnetic field for comb building and navigation.

Investigators have also discovered magnetite in a species of bacteria that live in seafloor sediment. When the sediment is stirred up, the earth’s magnetic field acts on the magnetite to align the bacteria in such a way that they propel themselves safely back into their seafloor home. Otherwise, they would die.

Many migratory animals​—including birds, turtles, salmon, and whales—​may also have a magnetic sense. However, they do not seem to rely on this sense alone but, rather, appear to navigate by a variety of senses. Salmon, for instance, probably use their strong sense of smell to find the stream of their birth. European starlings navigate by the sun; and some other birds, the stars. But as professor of psychology Howard C. Hughes observed in his book Sensory Exotica​—A World Beyond Human Experience, “we are obviously a long way from understanding these and other mysteries of nature.”

Ears to Envy

Compared with humans, many creatures possess amazing hearing. Whereas we can hear sounds ranging from 20 to 20,000 hertz (cycles per second), dogs can hear in the range of 40 to 46,000 hertz, and horses, between 31 and 40,000 hertz. Elephants and cattle can even hear in the infrasonic range (just below human hearing) to as low as 16 hertz. Because low frequencies travel farther, elephants may be able to communicate over distances of two or more miles [4 km]. In fact, some researchers say that we could employ such animals to give us an early warning of earthquakes and severe weather disturbances​—both of which emit infrasonic sound.

Insects also have a wide range of hearing, some in the ultrasonic range over two octaves above the human ear and others in the infrasonic range. A few insects hear by means of thin, flat, eardrumlike membranes, which are found on almost every part of the body except the head. Others hear with the aid of delicate hairs that respond not just to sound but also to the most gentle movements in the air, such as those caused by a human hand. This sensitivity explains why flies are so hard to swat!

Imagine being able to hear an insect’s footsteps! Such amazing hearing belongs to the world’s only flying mammal​—the bat. Of course, bats require specialized hearing to navigate in the dark and to catch insects by means of echolocation, or sonar.e Says Professor Hughes: “Imagine a sonar system more sophisticated than that found in our most advanced submarines. Now imagine that system is used by a small bat that easily fits in the palm of your hand. All the computations that permit the bat to identify the distance, the speed, and even the particular species of insect target are performed by a brain that is smaller than your thumbnail!”

Because precise echolocation also depends on the quality of the sound signal emitted, bats have the “ability to control the pitch of their voice in ways that would be the envy of any opera singer,” says one reference.f Apparently by means of the flaps of skin on the noses of some species, bats can also focus sound into a beam. All these assets contribute to a sonar so sophisticated that it can produce an “acoustic image” of objects as fine as a human hair!

Besides bats, at least two kinds of birds​—swiftlets of Asia and Australia and oilbirds of tropical America—​also employ echolocation. However, it seems that they use this ability simply to navigate in the dark caves where they roost.

Sonar at Sea

Toothed whales also employ sonar, although scientists have yet to discover exactly how this works. Dolphin sonar begins with distinct clicks, which are believed to originate, not in the larynx, but in the nasal system. The melon​—the bulb of fatty tissue on a dolphin’s forehead—​focuses the sound into a beam that “illuminates” a zone in front of the animal. How do dolphins hear their echoes? Not with their ears, it seems, but with their lower jaw and associated organs, which connect to the middle ear. Significantly, this region contains the same kind of fat as that found in the dolphin’s melon.

Dolphin sonar clicks are strikingly similar to a mathematical waveform called a Gabor function. This function, says Hughes, proves that dolphin clicks “approach a mathematically idealized sonar signal.”

Dolphins can adjust the power of their sonar clicks from a mere whisper to a cracking 220 decibels. How powerful is that? Well, loud rock music can produce 120 decibels, and artillery fire 130 decibels. Armed with sonar that is much more powerful, dolphins can detect things as small as a three-inch [8 cm] ball 400 feet [120 m] away and possibly even farther in quiet waters.

When you reflect on the amazing senses manifest in the living world, does it not fill you with awe and wonder? Humble, informed people usually feel that way​—which brings us back to the question of how we are made. True, our senses often pale beside those of certain animals and insects. Nevertheless, we alone are moved by what we observe in nature. Why do we have such feelings? And why do we seek not just to understand living things but to comprehend their purpose and to learn our own place among them?

[Footnotes]

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Insects Beware!

“Each day, just around dusk, a truly astonishing event takes place under the rolling hills near San Antonio, Texas [U.S.A.],” says the book Sensory Exotica​—A World Beyond Human Experience. “At a distance, you might think you saw an enormous black cloud billowing from the depths of the earth. However, it’s not a cloud of smoke that darkens the early evening sky, but the mass exodus of 20 million Mexican free-tailed bats from the depths of Bracken Cave.”

A more recent estimate places the number of bats exiting Bracken Cave at 60 million. Climbing up to 10,000 feet [3,000 m] into the night sky, they pursue their favorite meal, insects. Although the night sky must contain an overabundance of ultrasonic bat calls, there is no confusion, for each of these unique mammals is equipped with a highly sophisticated system for detecting its own echoes.

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Bracken Cave

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Courtesy Lise Hogan

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Mexican free-tailed bat​—sonar

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© Merlin D. Tuttle, Bat Conservation International, Inc.

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Honeybees​—sight and magnetic sense

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Golden eagle​—sight

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Ray​—electric sense

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Shark​—electric sense

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Starlings​—sight

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Salmon​—smell

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U.S. Fish & Wildlife Service, Washington, D.C.

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Turtle​—possibly magnetic sense

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Elephant​—low-frequency hearing

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Dog​—high-frequency hearing

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Dolphins​—sonar

Special Gifts That Make Us Unique

‘The scientist studies nature because he delights in it, and he delights in it because it is beautiful.’​—JULES-HENRI POINCARÉ, FRENCH SCIENTIST AND MATHEMATICIAN (1854-1912).

POINCARÉ deeply admired the beauty of the natural world, particularly “that profounder beauty” of harmony and order that appeals to the scientific mind. However, one need not be a scientist to appreciate the beauty and order around us. Some 3,000 years ago, the psalmist David was greatly moved by the design manifest in creation​—especially the design of the human body. Hence, he prayed: “I shall laud you because in a fear-inspiring way I am wonderfully made. Your works are wonderful, as my soul is very well aware.”​—Psalm 139:14.

Such feelings of wonder and reverence are uniquely human, beyond the capacity of even the most intelligent animal. Yet, our interest in the natural world goes deeper still. Thoughtful people of all ages have asked: What is the source of the amazing design of living things? Indeed, why do living things exist to begin with? And what is our place in the scheme of things? Science and introspective reasoning cannot answer these questions. But the Bible, which is inspired by God, does provide truly satisfying answers.​—2 Peter 1:20, 21.

This ancient sacred book explains that our unique human traits are the result of our being created “in God’s image”​—meaning that we are capable of reflecting (albeit to a lesser degree) our Creator’s personality traits. (Genesis 1:27) So although we do not have the eyes of an eagle, we can display farsighted wisdom. Our hearing may pale in comparison with that of a bat, but we delight in conversation, music, and the pleasant sounds of nature. And while we lack an internal compass, by turning to God’s Word, the Holy Bible, we get the very best guidance for life.​—Proverbs 3:5, 6.

Our being created in God’s image also explains why we alone have a spiritual need. “Man must live, not on bread alone,” Jesus said, “but on every utterance coming forth through Jehovah’s mouth.” (Matthew 4:4) Do you regularly take in those refreshing utterances by reading the Bible?

When properly nurtured by God’s Word, our spirituality can expand our perceptions beyond the limits imposed by our physical senses. How so? By building up our faith. Genuine Bible-based faith enables us to “see” the invisible God​—as did Moses—​and also to discern His purpose for the future.​—Hebrews 11:1, 27.

A Glorious Future for Those Who “See” God

The Bible teaches that the Creator, Jehovah God, loves the earth and all its living creatures, especially God-fearing humans. Hence, he promises to bring an end to all the wicked, including those who are greedily “ruining the earth.” (Revelation 11:18; Psalm 37:10, 11; 2 Thessalonians 1:8) Thereafter, he will grant eternal life to people who love and obey him. What is more, they will help turn the entire planet into a paradise bursting with life. What a wonderful prospect!​—Luke 23:43.

Imagine what you will be able to do and discover when life and good health are unlimited! “Nature,” wrote one scientist, “will always have novelty, richness, and beauty that can never be exhausted.” The Bible puts it this way: “Everything [God] has made pretty in its time. Even time indefinite he has put in their heart, that mankind may never find out the work that the true God has made from the start to the finish.”​—Ecclesiastes 3:11.

How can you be part of the Paradise described in the Bible? By learning about God’s purpose now and by acting on what you learn. “This means everlasting life,” Jesus said, “their taking in knowledge of you, the only true God, and of the one whom you sent forth, Jesus Christ.”​—John 17:3.